Ripple is the catchall name for the cryptocurrency platform, the transactional protocol for which is actually XRP, in the same fashion as Ethereum is the name for the platform that facilitates trades in Ether. Like other cryptocurrencies, Ripple is built atop the idea of a distributed ledger network which requires various parties to participate in validating transactions, rather than any singular centralized authority. That facilitates transactions all over the world, and transfer fees are far cheaper than the likes of bitcoin. Unlike other cryptocurrencies, XRP transfers are effectively immediate, requiring no typical confirmation time.
Ripple was originally founded by a single company, Ripple Labs, and continues to be backed by it, rather than the larger network of developers that continue bitcoin’s development. It also doesn’t have a fluctuating amount of its currency in existence. Where bitcoin has a continually growing pool with an eventual maximum, and Ethereum theoretically has no limit, Ripple was created with all of its 100 billion XRP tokens right out of the gate. That number is maintained with no mining and most of the tokens are owned and held by Ripple Labs itself — around 60 billion at the latest count.